Heavy-duty road vehicles such as straight trucks and semi-tractor trailers are customarily equipped with a powerful diesel engine that mechanically drives not only the vehicle but also a generator or alternator and various accessory pumps for braking and steering systems. In a typical configuration, the engine output shaft is coupled to the vehicle drive wheels through a manual or automatic transmission and one or more differential gearsets, and a power-take-off or belt-drive is used to drive the accessory systems. While this configuration has become standardized in the trucking industry, it necessarily results in vehicles that are costly, overly-heavy and only marginally efficient. For example, the drivetrain weight is very heavy, the engine frequently runs at high fuel consumption operating points, and the accessory pumps and motors are driven continuously and at variable engine-related speeds. Certain of these drawbacks have been addressed in industrial and heavy-duty off-road vehicles through the use of hydrostatic drive arrangements, and there is at least one company (Superdrive Inc., Piqua, Ohio) in the business of retrofitting heavy-duty road vehicles with hydrostatic drives. However, the cost and efficiency gains could be further improved in vehicles originally designed and manufactured to utilize hydrostatic drive arrangements.